[Rockhounds] Coating on rocks near Globe, AZ

[Al Balmer]

On Tue, 25 Nov 2008 13:33:48 -0600, "Jim Murowchick"
<murowchickj at umkc.edu> wrote:

>We just found one such piece in a donated collection--the substrate looks
>like it is a chunk of mortar or concrete (though not much can be seen)
>coated with what was labeled as malachite. When we ran the XRD, it turned
>out to be brochantite and nothing else.  I don't think there was a location
>given with the sample, though.
>
Interesting. Under a somewhat fuzzy 30X stereo microscope, this looks
much like the photo of brochantite under micro quartz overgrowth shown
at http://www.mindat.org/gallery.php?min=779 . There are definitely
tiny white crystals as well as the green. The green doesn't look like
needles, though. Can't say for sure - this microscope was a good one
40 years ago, but has been neglected.
>
>Jim Murowchick
>UMKC
>Kansas City, MO
>
>
>On 11/25/08 11:26 AM, "R. Peter Richards" <rpr at heidelberg.edu> wrote:
>
>> The obvious guess would be malachite, but there are several other
>> copper secondary minerals that would be possibilities.  Given your
>> observations, it would be an interesting experiment to turn over some
>> of the rocks, and see whether the green that is exposed goes away and
>> whether the previously exposed part becomes green.  And how long it
>> takes....
>> 
>> Pete Richards
>> 
>> 
>> On Nov 25, 2008, at 12:22 PM, Al Balmer wrote:
>> 
>>> On the west side of Globe, there's an area on the edge of a mine dump
>>> which has common stone covered with some green copper mineral. This is
>>> evidently a short-term process, since rocks half-buried in the dirt
>>> have the coating only on the covered part.
>>> 
>>> I'm curious as to what the coating is, and what the mechanism is.
>>> Anyone know?
>>> 
>>> -- 
>>> Al Balmer
>>> Sun City, AZ
>>> 
>>> -- 
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>> 
>> ___________________________________
>> R. Peter Richards
>> rpr at heidelberg.edu
>> Morphological crystallographer
>> 
>> 
>> 
>> 
>> 
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-- 
Al Balmer
Sun City, AZ